The present invention relates to a heat imaging apparatus including optical means as well as a heat radiation detector device located in the focal plane of the optical means. The heat radiation detector device may comprise one or several detector elements.
Heat imaging devices are, for example, used in the guidance control system of anti-tank weapons for the purpose of recognizing and combating heat radiating targets even when visibility is poor or at night under which conditions such targets may be hardly visible or not at all. The geometric as well as the thermal resolution of the heat imaging apparatus should be as large as possible for this purpose in order to recognize targets the temperature of which differs only slightly from the temperature of the environment. Further, the heat imaging apparatus should be also capable of recognizing or resolving temperature differences within the target itself in order to identify the target.
Generally, the resolution precision of such heat imaging devices is tested with the aid of test targets. These test targets comprise an arrangement of bars or beams, whereby adjacent beams display a temperature difference of about 2K.degree.. These tests targets are located at a given distance from the heat imaging apparatus or device. The resolution precision of the heat imaging device is then determined with regard to the objective whether the imaging device has recognized the target and if so, what information content has been identified.
The invention is based on the recognition that the optical means and the detector of such heat imaging devices have respectively different geometric resolution capabilities so that the two cooperating components also have different modulation transfer functions or characteristics. This fact has been recognized heretofore, for example, by Lloyd "Thermal Imaging Systems", Plenum Press New York 1975, pg. 99 and following pages. Said book provides the equations dealt with in the present disclosure so that for a more detailed discussion of said equations reference is made to said book. However, the conclusions reached according to the invention are not disclosed in said book by Lloyd.
The following discussion with reference to an actual example shall suffice to discuss the prior art.
Prior art heat imaging devices are required to resolve a test target having seven beams and a width of 2.3 meters as well as a temperature difference of 2K.degree. between adjacent beams, said resolution shall have a range as large as possible.
The resolution of optical devices is determined by, among other factors, the geometry of the imaging system. However, in heat imaging devices the resolution is strongly influenced by the relatively large wave lengths causing diffraction effects. Thus, a point located at an infinite distance does not appear as such on the optical axis of the heat imaging devices, rather, it is imaged as a diffraction figure in the form of the so-called diffusion circle. The diameter "d" of the diffusion circle depends on the wave lengths .lambda.and on the diameter "D" of the optical system of the heat imaging device. Thus, d is expressed as follows: EQU d=(.lambda..multidot.2.44)/D Eq. (1)
wherein "D" is expressed in miliradians.
The optical system frequently used in heat imaging devices has a diameter of 160 mm and a focal length of about 330 mm. For such an optical system the diffusion circle diameter is 0.15 miliradian for a wave length of 10.mu.. Expressed in length units the diameter of the diffusion circule is 50.mu.. Substantially all of the heat energy emanating from a point shaped heat source located at an infinite distance is concentrated in the diffusion circle as far as such heat energy is detected by the optical system. In this context the expression "substantially all" means about 85% of said heat energy emanating from a point source at an infinite distance.
In view of the above considerations all heat imaging devices of the prior art are so constructed that the individual heat detector elements correspond in their size at least to the diffusion circle in order to gather the maximum proportion of energy radiated from a target in order to detect such a target.
However, due to the above mentioned diffraction effects, the resolution capability of heat imaging devices is limited. For example, in accordance with the above mentioned dimensioning method it is possible to reduce the geometric resolution to 0.15 miliradians at the most for the selected device design.